The present invention is generally directed to fuel cells and more specifically to reversible fuel cells and their operation.
Fuel cells are electrochemical devices which can convert energy stored in fuels to electrical energy with high efficiencies. There are classes of fuel cells that also allow reversed operation, such that oxidized fuel can be reduced back to unoxidized fuel using electrical energy as an input.
One type of reversible or regenerative fuel cell is the solid oxide regenerative fuel cell (SORFC) which generates electrical energy and reactant product from fuel and oxidizer in a fuel cell or discharge mode and which generates the fuel and oxidant from the reactant product and the electrical energy in an electrolysis or charge mode. The SORFC contains a ceramic electrolyte, a positive or oxygen electrode and a negative or fuel electrode. The electrolyte may be yttria stabilized zirconia (“YSZ”) or doped ceria. The positive electrode is exposed to an oxidizer, such as air, in the fuel cell mode and to a generated oxidant, such as oxygen gas, in the electrolysis mode. The positive electrode may be made of a ceramic material, such as lanthanum strontium manganite (“LSM”) having a formula (La,Sr)MnO3 or lanthanum strontium cobaltite (LSCo) having a formula (La,Sr)CoO3. The negative electrode is exposed to a fuel, such as hydrogen gas, in a fuel cell mode and to water vapor (i.e., reactant product) in the electrolysis mode. Since the negative electrode is exposed to water vapor, it is made entirely of a noble metal or contains a large amount of noble metal which does not oxidize when exposed to water vapor. For example, the negative electrode may be made of platinum.
However, the noble metals are expensive and increase the cost of the fuel cell. In contrast, the prior art acknowledges that the negative electrodes cannot be made from a non-noble metal in a SORFC because such electrodes are oxidized by the water vapor in the electrolysis mode. For example, an article by K. Eguchi et al. in Solid State Ionics 86-88 (1996) 1245-1249 states on page 1246 that a cell with Ni-YSZ electrodes is not suitable for a solid oxide electrolyzer cell. The article further states on page 1247 that that a high concentration of steam (i.e., water vapor) caused the deterioration of a Ni-YSZ electrode and that a noble or precious metal negative electrode is preferred.